U.S. Pat. No. 5,896,949 describes an elevator installation in which the ride quality is actively controlled using a plurality of electromagnetic linear actuators. Such a system in commonly referred to as an active ride control system. As an elevator car travels along guide rails provided in a hoistway, sensors mounted on the car measure the vibrations occurring transverse to the direction of travel. Signals from the sensors are input to a controller which computes the activation current required for each linear actuator to suppress the sensed vibrations. These activation currents are supplied to the linear actuators which actively dampen the vibrations and thereby the ride quality for passengers traveling within the car is enhanced.
In the case where a large asymmetric load is applied to the car or where the car is poorly balanced, it would be necessary for one or more of the linear actuators to be powered continuously to overcome the imbalance. This continual energization would cause the actuator to heat up and, if left unchecked, could potentially lead to the thermal destruction of the actuator itself. It will be appreciated that the foregoing is only an example and that there are other cases where conditions imposed on the elevator car can similarly lead to overheating.
A conventional solution to this problem is to incorporate a bimetallic strip into the actuator to control its energization. Accordingly, when the temperature of the actuator rises to the predetermined activation temperature of the bimetallic strip, the bimetallic strip within the actuator would break the energization circuit and the respective actuator would be de-energized until its temperature falls to below the predetermined activation temperature of the bimetallic strip. It will be appreciated that at this switch-off point there would be an instantaneous deterioration in the performance of the active ride control, system since a force would no longer be generated by the effected actuator to stabilize the elevator car. Furthermore, this deterioration in performance would be immediately perceptible to any passengers traveling in the elevator car and would therefore defeat the purpose of, and undermine user confidence in, the active ride control system.